A system of this type is, e.g., described in EP 0027519 as an on-vehicle automatic loudness control apparatus. This system contains a microphone producing an output signal representative for the combined wanted signal level from the loudspeaker and the unwanted noise from motor, tires, wind, etc. present in the passenger space of a car. The system further contains a summing circuit adding the modified (squared) signals from the audio input and the microphone, one of which with reverse polarity. It is the object of the described system to provide automatic volume control depending on the level of disturbing noise. The system described in the above-mentioned EP 0027519 works independently from the frequency or frequency range of the wanted audio signal or the unwanted noise.
Another similar system is described in U.S. Pat. No. 5,077,799. This system contains a microphone producing a signal representative for only the ambient noise level. The microphone output signal is used to generate a control voltage for a variable resistance device. The control voltage circuit has different time constant elements for controlling volume up or down. Also, this circuit does not work depending on the frequency or frequency range of a wanted input signal or the ambient noise signal.
A further similar system is described in DE 4021969, which can be explained as a refinement of the solution as described in EP 0027519, before summing microphone and audio signal are filtered and two different signal paths are provided, one for the (normal) wanted audio signal and a parallel one for an effect sound derived from the wanted audio signal.
Also, this system works independently from the input audio frequency or frequency range and the ambient noise frequency or range.